A high definition television (hi-vision) displays a highly finely defined television picture on a wide screen having a large display area so as to provide novel attractive features, such as a strong appeal and a feeling of real presence on the site, which cannot be exhibited by the current conventional television broadcasting and is now favored with a growing interest in various countries.
As one of methods for attaining bandwidth compression of this high definition television signal, a method called MUSE (Multiple such-Nyquist Sampling Encoding) has been proposed by the Japan Broadcasting Corporation (NHK). The details of this MUSE are described in (Yuichi Ninomiya et al, "Satellite Broadcasting Channel No. 1 Transmission System (MUSE) for High Definition Television", The Institute of Electronics and Communication Engineers of Japan, Technical Report, IE84-72, 1984).
The outline of this transmission system is shown in FIG. 3. At a transmitting side, a picture signal obtained by a high definition camera or a VTR 31 is converted by an A/D conversion circuit 32 into a digital signal which is, after being subjected to sub-sampling in a sub-sampling circuit 33 and passed through a transmission matching filter 34, converted into an analog signal by a D/A conversion circuit 35, and the analog signal is transmitted after FM modulation by an FM modulator 36. At a receiving side receiving this signal, the signal is demodulated by an FM demodulator 37 and converted by an A/D conversion circuit 38 into a digital signal which is, after being subjected to sub-sampling in a sub-sampling circuit 39 and then processed by a two-dimensional interpolation circuit 310, converted into an an analog signal by a D/A conversion circuit 311, and this analog signal is displayed on a high definition display 312.
In the system, a horizontal synchronizing signal as shown in FIG. 4 is added to all the lines (1125 lines), and a vertical synchronizing signal as shown in FIG. 5 is added, as synchronizing signals.
FIG. 2 shows the basic structure of a prior art synchronizing signal processing circuit incorporated in a receiver. A signal applied to an input terminal 21 is demodulated by an FM demodulator 22 and, after the bandwidth is limited by a low-pass filter 23, the signal is converted into a digital signal by an A/D conversion circuit 24. A phase detection circuit 25 generates the result of computation of (the level at a point b+the level at a point c)-2 x (the level at a point a) in the horizontal synchronizing signal shown in FIG. 4, as an output representing the phase error of the synchronizing signal. Then, this signal is integrated by an integration circuit 26 so as to absorb any sharp change. A voltage-controlled oscillator (VCO) 27 generates a sampling clock applied to the A/D conversion circuit 24, and the output of the integration circuit 26 controls the phase of the sampling clock.
The vertical synchronizing signal (the frame pattern) shown in FIG. 5 is inserted in a picture signal corresponding to one horizontal period in one frame, and in the vertical synchronizing signal, 17.5 pairs of rectangular frame pulses having a pulse width of 4 ck at 16 MHz are repeated at a 100% level, where "ck" is a signal of higher frequency. A frame pulse detection circuit 28 detects the vertical synchronizing signal so as to provide a reference pulse for a timing generation circuit 29. On the basis of this frame pulse, the timing generation circuit 29 generates various timing pulses for a signal processing circuit 210. The signal processing circuit 210 carries out conversion inverse to the bandwidth compression etc. made at the transmitting side and, on the basis of the luminance signal and color difference signals, converts the signal into the red (R), green (G) and blue (B) signals, and a D/a conversion circuit 211 converts these signals into an analog signal (R, G, B) so as to display the picture on a high definition display 212.
However, with such a prior art structure, noise and/or a deformed picture may be displayed on the high definition display when the input picture signal is not a high definition television signal (a MUSE signal) or when transmission of a high definition television signal (a MUSE signal) is interrupted.